Without carrier aggregation, cellular data can only be transmitted over one frequency block at a time. Carrier aggregation allows data transfer to occur over multiple frequency blocks simultaneously.
You can think of it by analogy with filling a bucket with water. A bucket being filled by multiple hoses will fill faster than a bucket being filled by a single hose.
Carrier aggregation is only possible when both a cellular network and a device on the network support it. Among phones that support carrier aggregation, there can be differences in how many frequency blocks can be combined.
Under the current LTE specifications, a maximum of five frequency blocks may be combined through carrier aggregation.