Simplest way to concatenate two strings in robot framework .?

You can use Catenate from BuiltIn.

Example from docs:

${str1} =   Catenate    Hello   world   
${str2} =   Catenate    SEPARATOR=---   Hello   world
${str3} =   Catenate    SEPARATOR=  Hello   world
=>
${str1} = 'Hello world'
${str2} = 'Hello---world'
${str3} = 'Helloworld'

Catenate is the usual way to go with strings, as pointed in the other answer.
Alternative option is to use just Set Variable:

${a}=    Set Variable   First
${b}=    Set Variable   Second

${c}=    Set Variable   ${a}${b}
Log To Console    ${c}    # prints FirstSecond

${c}=    Set Variable   ${a} ${b}
Log To Console    ${c}    # prints First Second

${c}=    Set Variable   ${a}-/-${b}
Log To Console    ${c}    # prints First-/-Second

The explaination is that the RF processing of any keyword's arguments - Set Variable including, goes through substituting any variable with its value. E.g. for this call:

Set Variable   ${a}-/-${b}

What roughly happens is "the end value is the value of variable a-/-the value of variable b".

In Variable part, I used the most simple interpolation

${a}   Hello
${b}   World
${c}   ${a}${b}

If you need to assign the result to a variable that you use only once, you could instead do an inline expression e.g. using Python's str.join(), directly where you would use that variable. Catenate ultimately uses str.join().

${a}  Set Variable  Hello
${b}  Set Variable  World

Some keyword  ${{ " ".join([$a, $b]) }}
Some keyword  ${{ "\n\n".join([$a, $b]) }}

Note: This applies to RF 3.2 and higher.