If you don't understand, you certainly shouldn't send more money.
Normally with a pension the big plus is that you get tax relief so if you pay 20% tax, you can pay £80 and HMR&C will give back £20 so that £100 goes into the pension. The minus is that you can only get at the money when you retire and then only through an annuity (there are a few exceptions - you can get part of the pension pot as a lump sum, for example). Another minus is that some pension schemes have high charges. Another minus is that annuity (and other pension payments) are taxable.
If you pay off your mortgage it'll show clearly where the money has gone on the annual statement. When the house belongs to you, it is yours to do with as you wish. Should you then wish to swap the house for a VW Camper and a hugh stash of hash, then that's your choice to make.
Without knowing the details of the pension scheme, it's impossible for anyone to say what is best other than to say that, if you don't understand it, don't do it.