what exactly is a brane? i know that in order to get the equations of motion of a relativistic string by varying the Nambu-Goto action one has to impose a Neumann boundary condition and a Dirichlet boundary condition which yield the condition that the string's endpoints have to be attached to an object called a D-brane (D for Dirichlet, if i'm not wrong). I've been told these are like hypersurfaces which generalize strings for arbitrary dimensions. this means they have mass/energy/momentum, a size, a tension, and they could also have charges, right? then they would have to interact at least via gravitation. Does this mean they are like particles? One could never guesss that only by the Dirichlet conditions. in fact when i was following the derivation, i initially thought that these boundary conditions implied the endpoints of the string to be confined to a point/plane on spacetime, not that they were attached to "another particle" or "another energy-carrying thing" how does one obtain the interpretation of the Dirichlet condition as defining a brane?
TL;DR: could you explain (if possible ELI5) what exactly is, physically, a brane and where does it come from?
