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.

Take notes next time. This is a bit of gibberish I'm afraid and could possibly mean a lot of things. Call them back with pencil in hand. Bad valves and gear shifting are separate issues. The suspicion here is that you have low compression in a couple of cylinders and they won't know why (valves or head gasket) until they disassemble things. What this means to you is $$$$

Valves - are in the cylinder head and open and close as needed to allow in fuel and air and allow out exhaust gasses. If damaged or operating incorrectly then this could lead to low compression on the cylinder(s) in question.

Blown head gasket - gasket seals between the cylinder block and the cylinder head. If this fails then this too can lead to loss of compression in some or all cylinders

You can't compression test a valve

Compression tests are carried out with the car in neutral in a workshop with the engine running and not in any gear

I'm afraid that due to your lack of knowledge this question is partly gobbledegook but I hope the facts above will help you to understand part of what they are telling you.