LISA (evolved Laser Interferometer Space Antenna), a space-based detector with million-kilometer long arms will detect low-frequency gravitational waves that cannot be measured by ground based gravitational wave detectors.
The relative distance change between two freely falling bodies caused by a passing gravitational wave is exceedingly small, because spacetime is an extremely stiff elastic medium. For example: gravitational waves of a typical white dwarf binary at a distance of 50 pc create a periodic change of only 10-10 m in distance between two test masses, separated by a sufficiently large distance.
A suitable instrument for measuring such small length changes over a broad band of low frequencies is a laser interferometer with an arm length as large as possible and long integration times, the primary impetus for a space-borne detector.
Hence LISA-type detectors can be thought of basically as a Michelson interferometer in Space with an arm length in the order of million kilometers. This arm length allows observation of most of the interesting sources of gravitational waves.