I looked for knowledge searching part on the internet and I could understand the meaning of the above sentence.

>>>I searched the internet to find out the explanation and I was able to find the meaning of the sentence above.

Please refer to below paragraph.

>>>Please refer to the paragraph below.

First of all, if a person is on the ladder, a walk under the ladder might endanger.

>>>First of all, if a person is on the ladder, a walk under it is dangerous because some ladders are not safe.

Some ladders are not safe.
Second, a ladder up to a roof might suggest people are working on a roof.

>>>Second,a ladder up to a roof suggests that people are working on a roof.

 To walk under the ladder might endanger the person on the ground if things fall off the roof.