What it means
If an installer doesn't require admin rights, then it is installing software for the current user only, rather than system-wide.
What are the security impacts?
The software you are installing can only run under your own user account, so it has no way of modifying or damaging the system at superuser/admin level and affecting other users or system services. In this regard it is more secure than running a software installer that requires admin rights.
However, that doesn't mean the software itself can't do bad things, like any software, such as spying on you, spamming unwanted network traffic, messing with your files in your account, etc. None of this, however, is specific to the method in which software is installed.
Why system admins may not like it
It gets around any policy they may have around users installing software such as an approval process.
Is their concern justified?
Yes and no. While it isn't any more of a security risk to the system itself than anything you can already do in your user account (such as write scripts, run your own binaries or compile your own software), there are still some reasons IT departments may like to be notified or consulted anyway.
Some IT departments like to keep a record of what software exists on users' computers for the purpose of auditing. If the user installs software without IT knowing about it, then that software can still do nefarious things such as attack computers over a network, send spam, use up lots of resources on the machine, leak confidential documents and so on. So even though it may not be able to risk the system integrity for other users on that system, it still may perform things that are unwanted.
Even well-meaning software may have vulnerabilities which can cause problems if not kept up to date. If an IT department knows about software installed on users' systems it can ensure they get kept up to date.
What this all boils down to is that there is no inherent security risk in an installer that installs for a single user and doesn't use admin rights. What an IT department may be concerned about is simply the act of installing software without notifying them.