Salesforce allows only up to five batch jobs to run at a time, i.e. to be in a “Processing” status. When this limit is hit, jobs that are submitted are held in the Apex Flex Queue until resources become available to process them. Jobs in the flex queue sit in a “Holding” status until they are released for processing. The flex queue will only hold up to 100 jobs at a time. Once this limit is reached, you will see this error when a batch apex job is submitted.

Apex Governor Limits

Maximum number of batch Apex jobs in the Apex flex queue that are in Holding status – 100
https://developer.salesforce.com/docs/atlas.en-us.232.0.salesforce_app_limits_cheatsheet.meta/salesforce_app_limits_cheatsheet/salesforce_app_limits_platform_apexgov.htm

Up to 100 batch jobs can be in the holding status. When system resources become available, the system picks up jobs from the Apex flex queue and moves them to the batch job queue. The status of these moved jobs changes from Holding to Queued. Queued jobs get executed when the system is ready to process new jobs and they will then go to InProgress status.
https://help.salesforce.com/s/articleView?id=000334068&type=1