Strawberry plants multiply by runners (also known as stolons). At points along their length they put out adventitious roots i.e. roots that form from non root organs, in this case the runner. At each such node a new plant is created, in fact a clone- the genetic material is the same. This is a naturally occurring form of vegetative reproduction. 

 I planted six or seven plants in a bed on the allotment a couple of years ago and they spread very rapidly resulting in a strawberry patch about four metres by four metres. The bed was reclaimed from an area of couch grass, another rapid spreader which has come back strongly. Strawberries make a good ground cover but the couch will overwhelm them. 

 Accordingly I'm making my way through the bed with a fork uprooting the tufts of grass. Trouble is the grass and strawbs have intermingled so I have to tease out any strawberry plants within the clods. Laborious but in a sense this becomes an exercise in weeding and propagating at the same time. I'm replanting the ones I separate to further extend the patch.