Fit showing jump bid is a contract bridge convention usually played only in competitive bidding situations (i. e. where both sides are bidding).
After a player has opened in a suit or overcalled and opponents are also bidding, his partner's jump in a new suit shows length in the suit bid plus support for his partner's suit, normally 5-4/4-5 or better in the two suits. The bid is pre-emptive/competitive in nature, and shows a playing-strength for which you would otherwise have jumped to the next higher bid in your partners suit. The idea is to describe your hand more in order to help your partner judge whether to compete or not, in case opponents continue to bid.
Examples
3♣ shows ♥-support and ♣-suit and playing strength for a 3♥ pre-emptive bid. If, for example, West then bids 3♠, it is easier for North to judge whether to compete with 4♥ or not.
4♦ shows ♠-support and ♦-suit and playing strength for a 4♠ pre-emptive bid.
4♣ shows ♠-support and ♣-suit and playing strength for a 4♠ preemptive bid.
The exact situations where this convention applies or not applies may vary according to partnership agreements.
One variation is to agree that it only applies to non-game jump bids (so, for example: 1♠ - (2♣) - 4♥, is then natural showing ♥ only, and not fit-seeking with ♠-support).
The basic idea of Fit-jumps was widely popularized by Andrew Robson and Oliver Segal in the book Partnership Bidding in Bridge.