What does "Lost Mode Pending" mean?
It will remain pending until it connects to the Internet. 

It is recommended to change your Apple ID and other affected passwords, although if connected to Internet, the device should enforce the Lost Mode and protect your content.

If the person who has your phone has prevented it to connect prior to you having set up the Lost Mode information present on your iPhone is visible to them until it connects.

It will be forever pending if it is put on recovery mode before being connected to Internet, but it won't be usable because of Activation Lock. In this case your data will be erased.