Record Ids are the unique values for each record in Salesforce. Every Contact, Account, and every other object has a unique Id assigned to it automatically by Salesforce. In the background there are actually two Ids which are created, a 15 digit one and an 18 digit one.
15 Digit Ids are case-sensitive Ids, which Salesforce uses internally to maintain their database. Because they are case-sensitive it means that Contact A can have the Id 003000000000abc and Contact B can have the Id 003000000000ABC and these are seen as Unique from a Salesforce perspective.
18 Digit Ids on the other-hand, are not case-sensitive, and are also unique e.g. Contact A could have the Id 003000000000ABCCBA and Contact B could have the Id 003000000000CBAABC.
There are a couple reasons why you should use the 18 Digit Id record when loading data into Salesforce:
One way which you can view the 18 digit Id for an individual record is by looking in the URL, when you are viewing the record e.g.
If you want to view the 18 digit Ids in a report, then you will need to create a formula field on each object that you need to view the Id. Salesforce has a build in formula for you to use:
If you export records using dataimporter.io then you will automatically have the 18 digit Id, no need to create a custom field on each object.
While Salesforce Data Loader and other tools such as dataimporter, can recognize the 15 Digit Id, it is always better to use the 18 Digit Id. It will make sure that any data manipulation you need to do in Excel will work as expected, and that you data load processes cleanly.