How we change what others think, feel, believe and do
Asserting is stating something as if it were true.
The assertion may be made mildly, or may be with a certain force, for example using emphasis to strengthen what is being said.
When a person asserts something, they are also sending a message that they do not want to have an argument whether it is true or not.
I can definitely do this in five days.
That is the wrong method. This is a better way.
I have no money available for that.
Conversation is largely made up of a series of statements, all of which the speaker is asserting as truth except perhaps for qualifiers that admit possible untruth. Even then, the qualification is itself an assertion.
If what is said is false, then this can be either be because the speaker is deliberately lying, or because they genuinely believe what they are saying and are simply mistaken.
People may make assertions without being particularly confident in what they are saying. This can be because they think they will be more successful if they are assertive. Often, assertions are used simply because it is quicker than giving proof or rationale for everything that is said.