What is a LUD - Last Update Date
Whenever you a file a petition (such as I-485, Adjustment of Status) with USCIS, you get a Receipt Notice (Notice of Action - Form I797-C) that has a 'Receipt Number'. You can use this receipt number to check the status of your case at the USCIS web site.

Visit Online USCIS Case Tracking website.

You can enter the receipt number on the web site to check the status.

Alternatively, you can create a customer portfolio account that tracks one case (single applicant) or a representative portfolio account that tracks multiple cases. Representatives (persons that represent many cases such as lawyers, charitable groups, or corporations) who require regular access to status information about multiple cases find it convenient to create a representative portfolio account. The services provided are identical either way, except Representatives may also enter their own internal office tracking number with each receipt number. Additionally, you can also choose receive automatic case status updates via email.

Whenever there is a change (or sometimes no change) in your status, last updated date changes in your case status. LUD stands for Last UpDate or Last Update Date.

There are two types of LUD:

"Hard" LUD:
Your case status changes when USCIS sends you something (receipt notice, RFE, approval notice, notice of intent to deny, card, or other documents), and when they receive something you sent them (application, request for premium processing, answer to RFE, or other documents). Status can also change when USCIS makes a decision without sending you something as well. You will get an email notification for such changes.

"Soft" LUD:
There are many other internal activities occurring between the time the USCIS receives your application and it gets approved/rejected such as your fingerprints get added, your name check results come back from FBI, the petition gets prepared for approval etc. These are all internal changes to USCIS and there is no way to exactly know all the reason for soft LUDs. You will not get any email notification for such changes as it does not change the status of your petition.

If you would like to find out about all possible soft LUDs, you will have to log into your account at least once every business day. However, it is not necessary and you are more likely to get paranoid about it rather than get anything useful out of it.