Lawns are like most things if you maintain them and do not let them get patchy and run down they are strong and healthy and weeds will find it pretty hard to gain a foot hold.
Having said that your lawn is basically under attack 24/7 from the threat of weeds and foreign grasses and are usually carried in by bird droppings, pets, humans’ boots and particularly the wind
If you do find that your lawn is becoming infested by weeds then you will have to identify them and use a selective weed killer to remove them, we would advise for this job that you use a trained professional, if your lawn is very old and fibrous with a heavy presence of weeds than it is probably best to cut your losses and replace the lawn, after all turf is one of the cheapest surfaces on the plant by the sq metre and has not really gone up in price in real terms for 20 years.