That means that you change the “starting point” of the algorithm to be 7. And it will move from there and generate random numbers.
If later you set the starting point again to 7 and ask for random number, you will get the same !
import torch

print("Print 5 random numbers")
print(torch.rand(5))

print("Set the starting point to 15678 (this could be any integer number)")
torch.manual_seed(15678)

print("Print 5 random numbers")
print(torch.rand(5))

print("Print 5 more random numbers")
print(torch.rand(5))

print("Set the starting point to 15678 again")
torch.manual_seed(15678)

print("Print 5 random numbers")
print(torch.rand(5))