The traditional pile fleece is much-maligned these days. In terms of warmth to weight ratio, it does not top the Reddit ultralighter's spreadsheet, but the price is right and the weight difference compared to a newer grid fleece or alpha fleece is not massive. If you're just getting started hiking and backpacking, this is my top pick. We used to call this a 100-weight fleece. They last forever, too. I've been hiking with some version of a half-zip pullover fleece for 30 years now (I still have and use a Marmot version from about 1995). It just works.
Like all fleece jackets, Decathlon's HM 100 Hiking Fleece keep you warm by trapping body heat. Unlike down, it stays warm even when damp or downright wet (though wearing wet fleece is an unpleasant experience even if you are warm). It is breathable enough to help move moisture away from your base layer. It will also dry while you're wearing it. This is a minimalist's dream jacket, there are no thumb loops, no pockets, and no hood. It doesn't even have elastic on the cuffs. I find the lack of features to be a feature, but if you really must have more bells and whistles, Decathlon also makes the MH 120 Hiking Jacket which has a full zip and pockets, but weighs in at 12 oz for a men's medium (vs 7.9 oz for the MH100).
Grab a good base layer, a good puffer jacket, and a good rain jacket to go with this, and you'll be good to go in most three-season hiking environments.