Conversions represent the conversion goals you setup for a particular landing page.
(You can find the conversion goals tab in the top right corner of the page builder)
Leads are actual form submissions.
Now these number don't always match up 1 to 1 due to a variety of reasons:
You can have more than 1 conversion event on your landing page (ex. Form Submission as well as clicking on a specific link)
You can have a visitor fill out the form multiple times which would count as 1 conversion but will add 2,3,4 leads depending on how many times they submit the form.
You can also have visitors triggering a conversion by clicking on a link but not filling out a form. This would increase your conversions but won't add new leads.
Ideally, you would have a single goal for each of your landing pages and tracking only what matters. All other, secondary metrics, you can track through Google Analytics.