  MeerKAT's large number of antennas, spanning 8 km with a densely packed 1 km
core, create a powerful instrument for wide-area surveys, with high sensitivity
over a wide range of angular scales. The MeerKAT Galaxy Cluster Legacy Survey
(MGCLS) is a programme of long-track MeerKAT L-band (900-1670 MHz) observations
of 115 galaxy clusters, observed for $\sim$6-10 hours each in full
polarisation. The first legacy product data release (DR1), made available with
this paper, includes the MeerKAT visibilities, basic image cubes at $\sim$8"
resolution, and enhanced spectral and polarisation image cubes at $\sim$8" and
15" resolutions. Typical sensitivities for the full-resolution MGCLS image
products are $\sim$3-5 {\mu}Jy/beam. The basic cubes are full-field and span 4
deg^2. The enhanced products consist of the inner 1.44 deg^2 field of view,
corrected for the primary beam. The survey is fully sensitive to structures up
to $\sim$10' scales and the wide bandwidth allows spectral and Faraday rotation
mapping. HI mapping at 209 kHz resolution can be done at $0<z<0.09$ and
$0.19<z<0.48$. In this paper, we provide an overview of the survey and DR1
products, including caveats for usage. We present some initial results from the
survey, both for their intrinsic scientific value and to highlight the
capabilities for further exploration with these data. These include a primary
beam-corrected compact source catalogue of $\sim$626,000 sources for the full
survey, and an optical/infrared cross-matched catalogue for compact sources in
Abell 209 and Abell S295. We examine dust unbiased star-formation rates as a
function of clustercentric radius in Abell 209 and present a catalogue of 99
diffuse cluster sources (56 are new), some of which have no suitable
characterisation. We also highlight some of the radio galaxies which challenge
current paradigms and present first results from HI studies of four targets.
