Injecting the microbes deliberately during immunisation induce active immunity.(What does this line mean?)

Dear student,
Active immunity is the immunity which comes into action when antibodies are produced by the immune system of the organism by itself.
Sometimes microbes or infectious organisms are injected intentionally into the body of the host to observe and study the production of antibodies by the host to protect itself. By this the host becomes immune to the particular infection and this process is called as immunisation. 

Basically immunisation means making a host immune to a particular disease by injecting it intentionally. The concept of vaccine is based on this only.
So that lines simples means that sometimes during immunisation, active immunity can be naturally induced in the body of a host.

Regards.