It's another popular and in some ways more profitable and convinient strategy for making money from bitcoins and other cryptocurrencies. Comparing with simple buying & trading strategy mining is based on making calculations and building your own blockchain, these things will be the main ones for earning money with it. This strategy was very simple and very profitable in the beginning but later and now because of great competition you'll be able to make good money from this only with spending much energy for building your own unit and money for special software and hardware equipment, also its delivery and electiricity. Considering that you can simply invest more in all these things and get less in the end from your calculations, that's why cloud mining became popular too as a more convinient alternative :) It solves problems with that great costs because in its case you just sign up a contract with a certain company and don't need to build a unit with your own hands. On the other hand you won't have total control for all operations with your cryptocurrencies and fees sometimes are really high too, however it depends on a certain company again. And yeah, experienced miners recommend in this case only such companies as Eobot, Hashflare http://hashflarecode.com/ , Hashing 24, Genesis Mining and Hashnest which are the safest and most trustworthy in this case. There are much more other companies but they're sadly almost always a scam so be careful with making your choice.
Also having a start capital in the beginning is a big plus too.