Car compression issues valve 3 and 4?

I took my 2000 Honda Accord to a mechanic and they called back asking when the timing belt was changed and I said I don't know because I bought it as a used car recently and they said it could be a blown head gasket because they did a compression test and valve 3 and 4 ( or there were issues when switching from gear three to four )have issues. I don't really speak car so can someone explain what are valves, how does this all correlate.

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