For loop to delete rows from matrix

I have a number of matrices that are thousands of rows long. I am trying to keep rows that have a certain number in the second column, and delete everything else. I am trying to use a for loop with an embedded if statement but I don't know where to start. Any tips would be nice thank you.

Answer 1

[Jan]

M = randi([0, 5], 10000, 5);  % Some test data
P = M(M(:, 2) == 3, :);       % Keep only rows, which have a 3 in 2nd column
% [EDITED] Typo fixed, ",:" inserted. Thanks, Voss.

[Dimitri Haan]

This was very helpful but i was hoping to keep more rows of the matrix not just the one column. Basically it would be "if the value of column 2 equals 1, keep the entire row. If the value of column 2 doesnt equal 1, delete the entire row

[Voss]

P = M(M(:, 2) == 3, :);          % Keep only rows, which have a 3 in 2nd column