An English speaking question: What does "Let's say" mean?

For example.

let's say

English [lets se?] American [lets se?]?

For example; talk about; let us talk; let us Say

Example sentences:

1. Let's say you want to use a service, the mail service.

For example, let's say you want to use a service, an email service.

2. I?can?let?you?have?it?for,?well?let's?say100.?

I can sell this thing to you, um, for example Say give me ￡100.

3. You?could?learn?the?basics?in,?let's?say,?three?months.?

For example, you can master the basics in three months Knowledge.