Yes, it's a legal agreement but just nicer to look at!

The legal part: When the receiver actively choses to accept your quotation this is therefore, according to most jurisdictions, just as legally binding as if the receiver had signed a physical contract or, as is very common, answered with an “ok” via e-mail.

In practice: When your customer accepts your proposal, they will be reminded that they are confirming their acceptance of and understanding of the content and attachments of your proposal. It is therefore very clear for your customers that this is a binding agreement.