A trending market or asset is one where the price is tending to move in one direction of a period of time. For example an asset that is trending upwards means that it's price is going up for the time being.

Trending doesn't mean that it exclusively moves in the direction, however, as there will always be periods where the price reverses. But in general the reversals will be short lived and weak in comparison to the dominate trend.

Trending is also relative to time. An asset can be trending upwards for a day, but overall be trending downwards. For example, during the burst of the bitcoin bubble in 2018, there were periods where the price would trend upwards, but the longer term trend was one going down all the way from $20,000 to $3,000 per bitcoin.