A battery is a chemical reaction chamber where a chemical reaction that can be reversed results in current ( AMPS ) passing down the cables.
How much current can pass down the cables is proportional the the VOLUME of the chemicals ( called paste ) that is available to react.
The Voltage is a measurement of how far the reverse reaction ( called charging ) has gone,
IF there was just 1 molecule of active paste available to react & provide electricity the Volt meter will read 12.5 VOLTS or better.
However if you try to use that 1 molecules worth of electricity , it won't even cause a dull red glow in a globe.
Every time you reverse the reaction by charging the battery, a little bit of the paste is consumed so the volume of chemicals available gets less & less.
That is where your battery was at which is why the first test is to try and drive the starter directly from the battery using jumper leads.
CCA ( Cold Cranking Amps ) is a measurement of how fast the reaction can happen and for a mower you need between 200 & 300 ( or better )
AH is Amp hours which tells you how much chemical is in there to produce power, the higher the better and the more expensive.
