It is impossible to give a brief summary, that allows for an adequate asessment. Just the Reactions and criticism section wikipedia is about a ten minutes read.

Very broadly it is a gigantic infrastructure investment project by China, with the goal to strengthen and industrialize along certain trade routes through central asia and east africa connectinc China and Europe.

It could bring strong economic development especially for the central asian and african countries along the route but also further dependance on China. A particular point of criticism is the financing through China, where potentially entrapping debts for the countries are insured through access to natural ressources and the infrastructure itself. At worst a country could build a railway line or other infrastructure that ends up being owned and operated by China, employing chinese workers. On the other hand the criticism from the west needs to be also put into context of who gets to exert geopolitical control in areas like the middle east. The countries there could prefer chinese over US influence as US influence brought destabilisation and war.

https://en.wikipedia.org/wiki/Belt_and_Road_Initiative