Balance a chemical equation
Write the reaction the way you write it in your notebook and out come the smallest whole coefficients, the reaction ready to copy and the atom count side by side, which is the proof that it adds up. If it cannot be balanced it says so, instead of making numbers up.
How to write it, and how sloppy you are allowed to be
The arrow can be ->, →, = or ⇌, and a catalyst written over it is fine too, like --MnO2-->. The physical states (s), (l), (g), (aq) are read and set aside, because they play no part in balancing. Hydrates go however you like: CuSO4·5H2O, with a plain dot or with an asterisk. Subscripts can be the small ones copied out of a PDF: H₂O works fine. The electrons of half-reactions are written e-. And if you had already put some coefficients in, they get worked out again from scratch and you are told.
Charges, and the one place where this can go wrong
Ca2+ and MnO4- are built the same way (letter, digit, sign) but mean opposite things: in the first the 2 is the charge, in the second the 4 is a subscript of the formula. No rule tells them apart by looking, so nothing is guessed in silence: both readings are tried and the equation decides, then you are told which one was used. To be certain, write the charge with a caret: SO4^2-. And the charge is not an extra: it is one more row in the count, which is why half-reactions with electrons balance here like everything else.
The atom table is not an extra: it is the proof
A balancer that only hands you numbers is asking you to trust it. Under the reaction you get the atom count on the left and on the right, element by element, and the charge if there is one. Those numbers do not come from the calculation: the balanced line is read back from scratch, as if you had typed it yourself, and the atoms are counted from that. So the check catches printing mistakes as well. And if that count does not add up, you get no numbers at all: better to say it did not work out.
The answers that are not a number
These are the ones that matter most, and almost nobody gives them. If a substance gets a coefficient of zero it means it takes no part, and you are told so instead of getting a «0» in front of a formula. If it gets a negative coefficient, that substance is on the wrong side of the arrow. If the atoms add up and the charge does not, usually the electrons of a half-reaction are missing: that is what you are told, not a flat «impossible». And if the same substance sits on both sides, like the nitrogen in air, you are told it takes no part instead of getting numbers presented as final.
When there is more than one solution
It really happens: C + O2 → CO + CO2 has no single answer, because there are two different reactions inside it. A site that just spits numbers at you is hiding the problem, and those numbers even change if you write the substances in a different order. Here you are told, and you are shown the simplest reactions inside it: every other answer is a sum of them. Which one is right depends on the conditions of the reaction, which this tool does not know.
What this tool does not do
It is not a chemistry checker: it counts atoms and charge, it does not know whether the formulas you wrote exist at all. Balanced does not mean it happens: 2 H2O → H2O2 + H2 balances perfectly and never occurs. It does not guess products, it does not complete half-reactions with water and ions, it does not work out oxidation numbers and it does no stoichiometry: for weights there is Molar mass, for symbols there is Periodic table. It does not understand the names of substances, only formulas. And it does not fix your capitals, because Co is cobalt and CO is carbon monoxide.