Blending is a painting technique where two different colors are slightly mixed together when wet, giving a smooth transition from one color to the next. The transition color will be a product of the two blended colors (i.e if you are blending blue into a yellow, the transition color will be a green).
Blending required the paint to be wet, which is a problem when painting with acrylics as the paint tends to dry extremely fast. The blending technique is much more popular with oil painting, as the paint stays wet and versatile for much longer.
Blending is probably one of the most overused techniques by beginner painters, as they aim to create a very smooth and 'realistic' painting. However, too much blending can easily lead the painting having a very plastic and over-refined appearance.
Blending is much more effective when used sparingly when necessary. Brush strokes and colors should otherwise be left unaltered. Whilst up close the painting my look rough, as one steps back from the painting all the brush strokes and broken colors will optically merge and create the illusion of form.
Do not make the mistake of trying to make the painting look perfect from up close. If you are not sure what I mean, then take a look at some of John Sargent's paintings. Up close, they seem like a mess of bold strokes and colors. But as you step back the painting all comes together.