StringPadRight

StringPadRight["string",n] makes string be of length n, padding it on the right with spaces or truncating it if necessary. 

StringPadRight["string",n,"padding"] pads by repeating copies of the string padding.

StringPadRight[{s1,s2,…}] pads strings with spaces on the right to make them all the same length.

StringPadRight[{s1,s2,…},n,…] pads or truncates to make all strings of length n.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.