What does it mean when lambda is not entered?

Looking at the code today , See a code [lambda: gym.make("SimulatedEnv-v0", ) for _ in range(args.training_num)] ;
Yes lambda, But the colon is empty , No input , The output is the pile on the right . My first reaction was this lambda Is it possible not to
Later, I did an experiment , It's found that it can't be removed ！

def f(x):
    return x * 100

print([f(3) for i in range(5)])
print([lambda: f(3) for i in range(5)])

The former is a pile of results list, The latter is a function list：

[300, 300, 300, 300, 300]

[<function <listcomp>.<lambda> at 0x7fe7019acb80>, <function <listcomp>.<lambda> at 0x7fe7019acc10>, <function <listcomp>.<lambda> at 0x7fe7019acca0>, <function <listcomp>.<lambda> at 0x7fe7019acd30>, <function <listcomp>.<lambda> at 0x7fe7019acdc0>]