Strawberries are hopefully now in full fruit and you are having a good harvest.
If you see suckers appearing while the plant is still fruiting it is best to remove them. This will help the plants to put all their energy towards producing more delicious fruits. You can just snap these runners off with a sharp knife or a pair of scissors, or just pull it hard and they will come off (make sure you don’t pull the whole plant out though).
Suckers are young shoots that most of the strawberry varieties grow in order to propagate themselves, after they finished fruiting but sometimes they can form during fruiting or indeed throughout the whole growing season.
If you want to have more plants next year it is the best practice to leave the suckers after your plants finished fruiting. And when they become established and well rooted, after about 6 weeks from when they first appeared you can remove them from the mother plant.
They will easily take root if you grow them in raised beds or on the ground, but if you grow your strawberries in large pots or on covered ground you will need to help the runners take root. Use a small pot which you fill up with compost and place the runner in the pot and it will root down successfully.
You will easily spot where exactly the runner wants to root and just pin that bit in the pot. And in the autumn or early spring you can move the new young plants to their final position. Strawberries produce best in their second and third year in my opinion and after 4 years it is recommended to save the suckers and get rid of the older plants.