Simply put, a linear spring provides the same amount of resistance to compression for the entire length of the spring travel. For example a linear spring would provide 250lbs of resistance for every 1" of travel starting from a relaxed state until the coil was completely compressed.
On a progressive spring the amount of resistance changes as the spring is compressed. Most aftermarket springs are actually dual or triple rate springs where there is an initial soft spring rate, then a harder spring rate after a bit of compression. True progressive springs are more difficult to consistently manufacture than linear, dual or triple rate springs. It is also harder to tune the shock absorbers for non-linear springs since the spring rate changes somewhere or throughout the shocks cycle length.
The stock rear springs are progressive. You can tell a non-linear spring by either the change in coil spacing when relaxed or a change in coil diameter when relaxed. The stock rear springs are egg shaped, so the varying diameter indicates a progressive rate spring. The stock front springs are linear.
Here is a great example from Metal Cloak on its dual rate springs.
Metalcloak 3.5" Front Coil Spring, Dual-Rate, JK, Pair
The following companies produce progressive or multi-rate springs for Jeeps that I know of. There are likely others as well.
Polyperformance (Synergy), Rock Krawler, AEV, Metal Cloak.