You have a garden hose with one sprinkler on it, it works fine.
You add another sprinkler, then the first one only works 1/2 as good as it did. The more sprinklers you add the less well each one works. Electricity is similar.
You need to know what appliances or lights are on your circuit already before running power tools which take a lot of current generally speaking as opposed to other things like a TV or computer.
Any appliance that has a heater will take more current. Electric motors on larger power tools take a lot of current to start up then run on less. A corded electric drill will not take much current, but a table saw may take all the available current and trip the breaker. Generally speaking avoid extension cords...period.
A lighting circuit (no. 14 wire and 15 amps)will not have as much available current as an appliance circuit (no 12 wire and 20 amps). Power tools should be run on 20 amp circuits when possible.
If your lights dim, they are on the same circuit as the power tools, which is not recommended.
You can shut off the breaker to see what's on the circuit by plugging in a noisy appliance like a vacuum or radio turned up loud or have a helper tell you what went off. You can generally hear them vacuum from the basement if you are working alone.