With train fares being so high, the usual discount on off peak travel of one third is important if you qualify. You can find out what railcards are available here  But for many people the only railcard they can get is a Network Railcard, which anyone can buy, but which is only valid after 10.00 Monday to Friday and has a minimum Monday to Friday fare of £13 (more for a travelcard) and only covers the south east.

The answer may be to buy an annual season ticket, called a Gold Card.  This entitles you to one third off off peak travel in the south east from 9.30 without a minimum fare.

The cheapest annual season is from Newhaven Harbour to Newhaven Town which costs £172, which is a lot, but if you do a lot of shorter journeys the payment may be worthwhile.  On an off peak day return to Brighton you would save £1.75, so if you do that journey 100 times a year you would save money.

Additionally, you can nominate someone else to get a railcard they qualify for, for £10, an immediate saving of about £20.