This article is from the Car Audio FAQ, by Ian D. Bjorhovde (email@example.com) with numerous contributions by others.
An X-ohm stable amplifier is an amp which is able to continuously power
loads of X ohms per channel without encountering difficulties such as
overheating. Almost all car amplifiers are at least four ohm stable.
Some are two ohm stable, which means that you could run a pair of four
ohm speakers in parallel on each channel of the amplifier, and each
channel of the amp would "see" two ohms. Some amps are referred to as
"high-current", which is a buzzword which indicates that the amp is
able to deliver very large (relatively) amounts of current, which
usually means that it is stable at very low load impedances, such as
1/4 or 1/2 of an ohm. Note that the minimum load rating (such as "two
ohm stable") is a stereo (per channel) rating. In bridged mode, the
total stability is the sum of the individual channels' stability *Note