when I started my chess career the London system was the main "weapon" of choice for me. it has everything a chess player wants- a solid opening that doesn't require a lot of theory to learn.
the London system is a popular opening that can be used by all levels. the London starts with d4 but can be reached from nf3. it's a system-based opening. white can often follow the same moves and often ignores the opponent.
the London system starts after-
bf4 is the signature move of the London system. whites plan is quite simple- he wants to put his pawns on the dark squares and his bishop outside the pawn chain thus not restricting it like in most queen's gambit declined variations.
his main plan is to position his light-squared bishop on d3 often starting an attack on the kingside with the help of the minor pieces and the queen on c2. there is also a plan of playing e4/c4 and breaking up the center.
in the picture, we can see a dream position for the London system player.
one of the benefits for the London system player is that the king isn't castled yet. this gives white attacking prospects(pushing the h pawn up the board and so on).
let's look at a normal London system opening moves
black wants to undermine the white pawn center. playing c5 can help black weaken whites' strong c3,d4, and e3 pawn chain.
White's main point here is to strengthen his center.
black furthur pressures the center but we don't care at the moment.