What is dimension 'step' in dataset after a query to ERA5-Land monthly averaged data from 1950 to present

The python script produces a xarray dataset with dimensions 

Data variables:
    u10      (time, step, latitude, longitude) float32 ...
    v10      (time, step, latitude, longitude) float32 ...
    sp       (time, step, latitude, longitude) float32 ...

What is 'step' Cannot find any info...



import cdsapi

c = cdsapi.Client()

c.retrieve(
    "reanalysis-era5-land",
    {
        "format": "grib",
        "variable": [
            "10m_u_component_of_wind",
            "10m_v_component_of_wind",
            "surface_pressure",
        ],
        "year": "2017",
        "day": "09",
        "time": [
            "00:00",
            "01:00",
            "02:00",
            "03:00",
            "04:00",
            "05:00",
            "06:00",
            "07:00",
            "08:00",
            "09:00",
            "10:00",
            "11:00",
            "12:00",
            "13:00",
            "14:00",
            "15:00",
            "16:00",
            "17:00",
            "18:00",
            "19:00",
            "20:00",
            "21:00",
            "22:00",
            "23:00",
        ],
        "month": "09",
    },
    "download.grib",
)