Will my project felt? It depends. Felting is what happens when moisture meets agitation: the barbs on animal fibres lock together and the piece turns into one thick, fuzzy fabric, usually smaller than it was. Some people like that.
If you do not, keep agitation out of blocking. A light swish in the water is fine. Big temperature changes can set it off too, so use lukewarm or room temperature water.
Acrylic and superwash yarns do not felt. They can go through a cold cycle in the washing machine without any trouble.
1 Felting is moisture plus agitation
Felting happens when moisture and agitation come together. The little barbs on animal fibres lock into each other and the piece becomes one big fuzzy fabric. It usually shrinks your project too.
2 Keep the agitation out
If you are not felting on purpose, do not agitate the piece. Swishing it around lightly in the water is not enough to felt it, so a gentle wash is fine.
3 Keep the water lukewarm
Big changes in temperature can set felting off. Use lukewarm or room temperature water, not very cold and not too hot.
4 Acrylic and superwash will not felt
Acrylic and superwash yarns do not felt. You can agitate them, and even run them through a cold cycle in the washing machine, without them turning into a fuzzball.