what are vnav pth and vnav spd on the fma?

Hi everyone

I would like to ask about topic title. what are the differences between these two modes? when do they occur? what do they mean? they always show up on the fma ( flight mode annunciator). in addition tell the same about vnav alt, please. I think these 3 modes are only in subject to vnav. when i get this described I will see everything clearly so this very important thing to me which is related to flying 737-800 ngx, without it I am flying but I don't understand what is happening.

Pleaseee help me, any answer will be appreciated