CSI RS in a Nutshell
- What is it for ? It is a special type of Reference Signal being transmitted by gNB for UE to use in order to estimate downlink radio channel quality.
- In LTE, CRS (Cell Reference Signal) is usually used for this purpose but NR does not have CRS. You may consider CSI RS in NR as a counter part of CRS in LTE.
- Main difference between CRS(LTE) and CSI RS (NR) is that CSI RS should be configured by RRC to be transmitted (meaning that it is not mandatory signal) whereas CRS is always being transmitted.
- Another difference is that NR CSI-RS is designed with consideration for beamforming.
- There are many different options for the number of CSI RS ports : 1,2,4,8,16,24,32
- 1 port configuration (p1) is mainly used for TRS (Tracking Refernece Signal) and multi port configuration (e.g, p2 or higher) are mainly used for CSI measurement
- CSI RS can be transmitted in any OFDM symbols and in subcarriers in almost arbitrary (not 100% arbitrary but super flexibily) way as configured in RRC message.
Read more here: 
5G_CSI_RS Explanation
And here: NR - CSI RS Animation
LinkedIn: 