What is mean by this keywords ?.

[Hapu Arachchilage Tharindu Lakmal]

Hi, I'm new to MQL5 coding. Every time I type something in (Like function) these keywords are before the function name (bool, double, string) 
I curious to know what exactly mean by it. sometimes I've seen same function is in different formats like bool, double and string. 

Does it mean the data type of the return value of the function?  
if yes, how we can check later what kind a type we have used? (ex - ObjectGetInteger vs ObjectGetInteger)