Because your Landlord is paying you over $600 per year ($60 per month x 12 = $720), he is required to send you a Form 1099-MISC reporting the income. Form W9 is used to request your Social Security Number & Other identifying information needed in order to properly complete the Form 1099-MISC.
The income paid to you by your Landlord is considered "Self-Employment Income" and should be reported on Schedule C of your tax return. Be sure to keep careful track of any expenses you incur doing the Yard Work because you will be able to deduct them from the income.
See the TurboTax FAQ regarding Self-Employment Income for more details:
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