Below is an explantion of each point you list in your question.
[[ What is int i, j; ]]
Declares two variables named i and j, both of type int (Integer).
[[ What is char c: ]]
Declares a variable named c of type char, which holds a single character.
[[ What is float x ]]
Declares a variable named x of type float (floating point number).
[[ i=4 ]]
Assigns the value 4 to the variable named i.
[[ j=i+7; ]]
Assigns the current value of the variable named i plus 7 to the
variable called j. For example, if the current value of i is 4, then j
would equal 11 after this line runs.
[[ x=9.087 ]]
Assigns the value 9.087 to the variable named x.
[[ x=x*12.3 ]]
Multiplies the current value of the variable named x by 12.3 and
assigns the result back to the variable called x.
[[ std::cin>> integer1; ]]
Uses the stream extraction operator (>>) to place the value typed by
the user in a variable named integer1. std::cin means standard input.
[[ cout<<i << "," << j << ", " << x << "\n"; ]]
Uses the stream insertion operator (<<) to send the expression(s) on
the right to standard output (cout). The result sent to standard
output would be the value of i followed by a comma and a space,
followed by the value of j followed by a comma and a space followed by
the value of x followed by a newline (\n). So if the variables are set
i = 5
j = 10
x = 9.087
...then the output would be
5, 10, 9.087 (newline)
Please ask for clarification if you need additional information on these concepts.
An explanation of variable types like int, char and float, variable
declarations and assignments.
An introduction to stream operators