Execution time is the total time for which program used the CPU.
Time complexity on other hand gives us an idea of with what scale execution time will increase on increasing the input.
Suppose Algo 1 is having a Time complexity of O(1), and it takes 4s as execution time when the input is given is of length 100. This means even if the input given is of length 1000, it'll still take 4s.
similarly, if Algo 2 is of O(n) and takes 4s for input of length 100, on an input of 200, it'll take around 8s. time will linearly increase for this.