It has to be refreshed because its memory cells are designed around tiny capacitors that store electrical charges. These capacitors work like very tiny batteries and will gradually lose their stored charges if they are not re-energized. Also, the process of retrieving, or reading, data from the memory array tends to drain these charges, so the memory cells must be precharged before reading the data. Refresh is the process of recharging, or re-energizing, the cells in a memory chip. Refresh can be accomplished in many different ways, which is one of the reasons it can be a confusing topic.
Refresh rate is determined by the total number of rows that have to be refreshed in a memory chip.
Memory chips are designed for a particular type of refresh. For example, chips using 4K refresh will have about 4000 rows, which means that it will take about 4000 cycles to refresh the entire array. Chips using 2K refresh will have about 2000 rows, and chips with 1K refresh will have about 1000 rows. The capacity is the same, but the organization and refresh are different. The major refresh rates in use today are 1K, 2K, 4K, and 8K. The primary reason for these different types of refresh is decreased power consumption.